The Google Cultural Institute platform is open to non-profit institutions, museums, galleries, and archives with copyright-free or copyright-cleared content that they would like to share.
The New York Public Library offers a collection of searchable historical photographs online. Browse through their image collections, or search for a keyword.
The Web Gallery of Art is a searchable database of European painting and sculpture from 12th to mid-19th centuries. The site includes a database of fine arts museums, churches and a glossary of art terms.
AP Images is a large collection of historical and contemporary news and lifestyle photos. Some content is restricted but can be searched by keyword, though not printed.
An extensive collection of online photographs produced by the photography cooperative Magnum Photos. In Advanced Search limit by year and image type and search for your topic.